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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Tunneling in hydrogen-transfer isomerization of n -alkyl radicals
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Tunneling in hydrogen-transfer isomerization of n -alkyl radicals

机译：正烷基氢转移异构化中的隧穿

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The role of quantum tunneling in hydrogen shift in linear heptyl radicals is explored using multidimensional, small-curvature tunneling method for the transmission coefficients and a potential energy surface computed at the CBS-QB3 level of theory. Several one-dimensional approximations (Wigner, Skodje and Truhlar, and Eckart methods) were compared to the multidimensional results. The Eckart method was found to be sufficiently accurate in comparison to the small-curvature tunneling results for a wide range of temperature, but this agreement is in fact fortuitous and caused by error cancellations. High-pressure limit rate constants were calculated using the transition state theory with treatment of hindered rotations and Eckart transmission coefficients for all hydrogen-transfer isomerizations in n-pentyl to n-octyl radicals. Rate constants are found in good agreement with experimental kinetic data available for n-pentyl and n-hexyl radicals. In the case of n-heptyl and n-octyl, our calculated rates agree well with limited experimentally derived data. Several conclusions made in the experimental studies of Tsang et al. (Tsang, W.; McGivern, W. S.; Manion, J. A. Proc. Combust. Inst.2009, 32, 131-138) are confirmed theoretically: older low-temperature experimental data, characterized by small pre-exponential factors and activation energies, can be reconciled with high-temperature data by taking into account tunneling; at low temperatures, transmission coefficients are substantially larger for H-atom transfers through a five-membered ring transition state than those with six-membered rings; channels with transition ring structures involving greater than 8 atoms can be neglected because of entropic effects that inhibit such transitions. The set of computational kinetic rates were used to derive a general rate rule that explicitly accounts for tunneling. The rate rule is shown to reproduce closely the theoretical rate constants.
机译：使用多维，小曲率隧穿方法研究了传输系数和在CBS-QB3理论水平上计算出的势能面，探讨了量子隧穿在线性庚基自由基氢转移中的作用。将几个一维近似值（Wigner，Skodje和Truhlar和Eckart方法）与多维结果进行了比较。与在宽温度范围内的小曲率隧穿结果相比，发现Eckart方法足够精确，但实际上这种协议是偶然的，并且是由误差消除引起的。使用过渡态理论，对正戊基至正辛基中所有氢转移异构化的受阻旋转和Eckart传输系数进行处理，计算出高压极限速率常数。发现速率常数与可用于正戊基和正己基的实验动力学数据高度吻合。在正庚基和正辛基的情况下，我们的计算速率与有限的实验得出的数据非常吻合。在Tsang等人的实验研究中得出了一些结论。 （Tsang，W .; McGivern，WS; Manion，JA Proc。Comstt.Inst.2009，32，131-138）在理论上得到证实：较早的低温实验数据具有较小的指数前因子和活化能，可以考虑到隧道效应，与高温数据保持一致;在低温下，H原子通过五元环跃迁状态的传递系数比具有六元环的传递系数大得多。由于抑制这种转变的熵效应，可以忽略具有大于8个原子的过渡环结构的通道。计算动力学速率的集合用于导出明确解释隧道效应的通用速率规则。速率规则显示为紧密再现理论速率常数。

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